GCSE Chemistry revision

The development of the model of the atom

A simple model of the atom, symbols, relative atomic mass, electronic charge and isotopes

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The ancient Greek philosopher Democritus (around 500 BC) first proposed that all matter is made up of tiny, indivisible particles called atoms, separated by empty space.
This idea remained largely unchanged for approximately 2,300 years until scientists in the 1800s began to develop and improve upon it.

1. Democritus and Early Atomic Theory AQA

The ancient Greek philosopher Democritus (around 500 BC) first proposed that all matter is made up of tiny, indivisible particles called atoms, separated by empty space.
This idea remained largely unchanged for approximately 2,300 years until scientists in the 1800s began to develop and improve upon it.

In the early 1800s, John Dalton described atoms as solid spheres, proposing that different types of spheres corresponded to the different chemical elements.
Dalton's model was a significant step forward as it linked the concept of atoms directly to the elements we observe in chemistry.

In 1897, J.J.Thomson conducted experiments that proved atoms could not be solid spheres, leading to the discovery of negatively charged particles called electrons.
Thomson proposed the plum pudding model, in which the atom was described as a ball of diffuse positive charge with discrete electrons embedded within it.

3. Thomson's Plum Pudding Model (1897) AQA

In 1897, J.J.Thomson conducted experiments that proved atoms could not be solid spheres, leading to the discovery of negatively charged particles called electrons.
Thomson proposed the plum pudding model, in which the atom was described as a ball of diffuse positive charge with discrete electrons embedded within it.

Ernest Rutherford and his students fired positively charged alpha particles at a thin sheet of gold foil, expecting them to pass straight through if Thomson's model were correct.
Surprisingly, some alpha particles were deflected sideways and a small number bounced directly back, disproving the plum pudding model.
Rutherford proposed the nuclear model, suggesting that all positive charge is concentrated in a small, dense nucleus at the centre of the atom, surrounded by a cloud of negative electrons.

4. Rutherford's Gold Foil Experiment (1909) AQA

Ernest Rutherford and his students fired positively charged alpha particles at a thin sheet of gold foil, expecting them to pass straight through if Thomson's model were correct.
Surprisingly, some alpha particles were deflected sideways and a small number bounced directly back, disproving the plum pudding model.
Rutherford proposed the nuclear model, suggesting that all positive charge is concentrated in a small, dense nucleus at the centre of the atom, surrounded by a cloud of negative electrons.

A key problem with Rutherford's nuclear model was that it could not explain why the negatively charged electrons were not pulled into the positively charged nucleus, which would cause the atom to collapse.
This unresolved issue highlighted the need for a further development in atomic theory.

5. The Flaw in Rutherford's Nuclear Model AQA

A key problem with Rutherford's nuclear model was that it could not explain why the negatively charged electrons were not pulled into the positively charged nucleus, which would cause the atom to collapse.
This unresolved issue highlighted the need for a further development in atomic theory.

In 1913, Niels Bohr proposed that electrons orbit the nucleus in fixed energy levels called shells, in a similar way to how planets orbit the Sun.
The orbiting motion of electrons in shells explained why they do not spiral into the nucleus, solving the major flaw in Rutherford's model.
Bohr's model has been supported by many subsequent experiments and forms the basis of the atomic model used in GCSE Chemistry today.

6. Bohr's Atomic Model (1913) AQA

In 1913, Niels Bohr proposed that electrons orbit the nucleus in fixed energy levels called shells, in a similar way to how planets orbit the Sun.
The orbiting motion of electrons in shells explained why they do not spiral into the nucleus, solving the major flaw in Rutherford's model.
Bohr's model has been supported by many subsequent experiments and forms the basis of the atomic model used in GCSE Chemistry today.

Further experiments by Rutherford revealed that the positive charge within the nucleus is made up of small, discrete particles called protons.
James Chadwick later provided experimental evidence for the existence of neutral particles within the nucleus, which are called neutrons.
Together, protons and neutrons make up the nucleus of an atom, completing our modern understanding of atomic structure.

7. Discovery of Protons and Neutrons AQA

Further experiments by Rutherford revealed that the positive charge within the nucleus is made up of small, discrete particles called protons.
James Chadwick later provided experimental evidence for the existence of neutral particles within the nucleus, which are called neutrons.
Together, protons and neutrons make up the nucleus of an atom, completing our modern understanding of atomic structure.